1. Field of the Disclosure
The present disclosure relates generally to communication systems and, more particularly, to wireless communication systems.
2. Description of the Related Art
Wireless communication systems provide wireless connectivity to user equipment using a network of interconnected access nodes or base stations. Communication over the air interface between the user equipment and the base stations take place according to various agreed-upon standards and/or protocols. For example, the Third Generation Partnership Project (3GPP, 3GPP2) has specified a set of standards for a packet-switched wireless communication system referred to as Long Term Evolution (LTE). The LTE standards support access schemes including single-carrier frequency division multiple access (SC-FDMA). Multiple users can concurrently access the SC-FDMA network using different sets of non-overlapping Fourier-coefficients or sub-carriers. One distinguishing feature of SC-FDMA is that it leads to a single-component carrier transmit signal. The LTE standards also support multiple-input/multiple-output (MIMO) communication over the air interface using multiple antennas deployed at transmitters and/or receivers. The carrier bandwidth supported by LTE is approximately 20 megahertz (MHz), which can support a downlink peak data rate of approximately 100 megabits per second (Mbps) and a peak data rate of the uplink of approximately 50 Mbps.
User equipment may implement transceivers that include a transmitter for transmitting uplink signals towards the network and a receiver for receiving downlink signals transmitted by the network. Transceivers implemented in user equipment may communicate according to the SC-FDMA standards or protocols using different sets of non-overlapping Fourier-coefficients or sub-carriers. User equipment conventionally communicate with each other by transmitting signals over a communication path that originates at the transmitter of the first user equipment, travels over an uplink (or reverse link) to a receiver in a network base station, proceeds to a transmitter in the receiving base station or another base station, and is then transmitted over a downlink (or forward link) to a receiver in the second user equipment.
Device-to-device (D2D) communication refers to information exchanged between two wireless communication devices over a communication path that does not necessarily include the network in the communication path. For example, D2D communication allows two user equipments to communicate directly with each other over an air interface between the two user equipments without including any network elements in the D2D communication path. The D2D communication protocols can be used to support voice or data transmission on a one-to-one basis (unicast), on a one-to-many basis (groupcast) for applications such as push-to-talk (PTT), or on a one-to-all basis (broadcast). User equipment can also serve as relay stations for D2D communication between other user equipment. The absence of network support should not interfere with D2D communication and user equipment can perform D2D communication in either an in-coverage scenario (when user equipment are able to communicate with the network) or an out-of-coverage scenario (when user equipment are not in communication with the network).